This invention relates to a bar skiascope of a type having a lens slide for displacing a condenser lens in front of a lamp inside a skiascope housing.
A skiascope, also called a retinoscope in international parlance, is a device used to assess objectively defective vision of an eye, and to determine eyeglass lenses needed to correct the defective vision. A type of skiascopy, also called retinoscopy in international parlance, has been known for decades, and is used worldwide in the field of ophthalmology because it can be used without regard to location, and because of its accuracy.
In the field of skiascopy, a distinction is made between bar skiascopy (bar retinoscopy) and point skiascopy (point retinoscopy), with bar skiascopy being used more often than point skiascopy.
A bar skiascope comprises mainly an incandescent lamp, a condenser lens, and a semireflecting mirror. Light generated by the incandescent lamp is made divergent, parallel, or convergent by displacement of the condenser lens, and is deflected via the mirror as a light band onto an eye to be examined. An examiner looks through the semireflecting mirror coaxially along the axis of a light beam, and observes light reflected from a fundus oculi of an eye. As he does so, he executes oscillating motions with the bar skiascope, so that the eye is alternately illuminated and not illuminated. The direction of movement and speed of movement of the observed reflex provide information about the type of vision defect. By placing corrective lenses in front of the eye, the reflexes change, until ultimately a condition characterized as alignment is achieved, in which the eye is fully corrected.
Over time, various techniques have been developed for using bar skiascopes, which are characterized by various examination parameters. For example, an examination distance may be a set, unchanging value, or it may be changed intentionally during the examination. In addition, an illumination beam can be divergent at the start of an examination and become convergent as the examination progresses, etc. A direction of the light band can also be set at any value by rotating the incandescent lamp through almost 360.degree..
In a common application, a beam path is made parallel for a brief time during an examination, in order to determine an axial position of astigmatism, for example, or to check axial correction already achieved using supplementary lenses. To that end, an examiner projects a light beam of a bar skiascope onto a wall at as great a distance as possible, and focuses the light band into a bar image. This procedure is time-consuming, since the examination must be interrupted, and it is imprecise, because parallel adjustment is usually impossible at a distance of only a few meters, usually. However, an adjustment that is not quite exactly parallel has an immediate impact on results of the examination because of a requirement for accuracy that is necessary in this examination method. This may result in an inaccurate or even erroneous diagnosis.
In a known skiascope, a locking device is provided as a device to fix the beam in a parallel position. In this design, a lens slide for displacing the condenser lens engages in a locked position for achieving a parallel beam position. This does make setting the parallel beam position considerably easier, but this locking in position impedes sliding transition from convergent to divergent ranges, and vice versa as is, needed for other examination techniques. Furthermore, convergence and divergence cannot be adjusted in an immediate vicinity of the locking position.
It is an object of this invention is to provide a bar skiascope which allows the fixing of a beam in a parallel-beam position that overcomes the stated disadvantages of fixing devices known in the prior art.